1. Technical Field
The present invention relates generally to electro-optical structures having total-scattering and semi-transparent and totally-transparent modes of operation which are electrically-switchable for use in dynamically controlling electromagnetic radiation flow in diverse applications, such as electro-optical glazing structures, and also to improved methods and apparatus for producing such electro-optical structures in a large-scale and uniform manner, without defects or haze required applications such as switchable privacy window glazings.
2. Background Art
The use of windows in homes, commercial buildings, and automotive vehicles alike is very well known. The reasons for providing windows in such structures and systems are directly related to the functions they perform. For example, window structures provide for ventilation, lighting, a sense of spaciousness, as well as a way of making contact with the outdoors. Windows made of glazing (e.g. glass material) also permit selective transmission of electromagnetic radiation between the outdoors and the interior space of homes, commercial buildings, and automotive vehicles. While conventional forms of glazing serves many useful functions, such forms are not without problems.
An appreciation of the problems presented by the use of conventional glazing in windows, can be most easily attained by recognizing the nature and composition of electromagnetic radiation with which windows universally come in contact.
On a clear day at sea level, electromagnetic radiation is composed of 3% ultraviolet light (i.e. electromagnetic radiation in the UV band), 44% visible light (i.e. electromagnetic radiation in the visible band), and 53% infrared light (i.e. electromagnetic radiation in the IR band). In accordance with the laws of physics, 50% of all electromagnetic radiation produced is left hand circularly polarized (LHCP) while the other 50% thereof is right hand circularly polarized (RHCP). The total electromagnetic radiation striking a window surface is a combination of direct radiation from the Sun and diffuse radiation from the ambient environment. While electromagnetic radiation is broad-band in nature, it is the ultraviolet light component thereof which causes molecular decomposition in various types of plastic material and inorganic dyes, which results in color fading.
When electromagnetic radiation strikes a glass window, three different physical processes occur. Some of the radiant energy is transmitted through the glass; some of the radiant energy is reflected off the glass; and a small portion of the radiant energy is absorbed by the glass. The energy transmitted through the glass window is typically absorbed by furnishings or structures within the interior environment, and often becomes trapped therewithin causing an increase in interior temperature.
Depending on the season, electromagnetic radiation transmitted through glass windows can either mitigate or worsen the thermal loading imposed upon the heating and cooling systems associated with the glass windows. Consequently, during the hot weather season, it is highly desired to shield windows and sliding glass doors from electromagnetic radiation in order to lessen thermal loading upon cooling systems. During cold weather season, it is highly desired to expose windows and sliding glass doors to electromagnetic radiation in order to lessen thermal loading on heating systems.
In short, it is highly desired to selectively control the transmission of electromagnetic radiation through window structures at different times of the day and year so that thermal loading upon the heating and cooling systems of residential, commercial and industrial building environments can be minimized. By minimizing such thermal loading, power can be used in an economical manner to control the internal temperature of residential, commercial and industrial building environments. Achievement of this goal would impact the natural environment in a positive manner, while improving the quality of life.
With such objectives in mind, great effort has been expended in recent times to improve the ways and means of selectively controlling the transmission of electromagnetic radiation through window structures.
One approach to electromagnetic radiation control involves using a window shade to reduce the transmission of electromagnetic radiation through windows. The most popular type of shade is the window blind. However, as window blind is mounted within the interior of the building or transportation environment, electromagnetic radiation is allowed transmit through the window, raises the temperature within the internal environment, and thus increases thermal loading on cooling systems during the hot weather season. Also, the operation of window blinds requires mechanical or electro-mechanical controls which tend to be bulky and expensive to manufacture, install and maintain.
Another approach to electromagnetic radiation control involves the use of sun control films which are physically applied to the surface of glass windows in building and automotive vehicles alike. Presently a variety of different types of sun control film are marketed by various firms. Such electromagnetic control films can be categorized into one of the three basic categories, namely: high reflectivity film; heat saving or winter film; and fade protection film.
High reflectivity electromagnetic films are most effective at blocking summer heat. The higher the reflectivity of electromagnetic film, the more effective it will be at blocking electromagnetic radiation. Electromagnetic reflectivity film having a silver; mirror-like surface is more effective in blocking electromagnetic radiation than the colored, more transparent films. Electromagnetic reflectivity films can lower the U-value of glass by more than 10%. Notably, in climates having long heating seasons, the use of high reflectivity film prevents using the winter sun to warm the interior of buildings during the cold weather season, and thus lessen thermal loading on building heating systems.
Heat-saving or winter films are designed to reduce winter heat losses through glazing. These films can lower the U-value of glass windows by more than 20%.
Fade-protection films are designed to filter out ultraviolet rays. Ultraviolet rays cause about 60-65% of color fading in most home furnishing fabrics and automobile dash boards.
While electromagnetic radiation control films of the types described above can be used to control heat and glare, eliminate sun damage, and to a lesser extent, reduce visibility into buildings during the daytime. The major disadvantages thereof are reduction in interior light, loss of visibility, and extra care required in cleaning. Moreover, prior art electromagnetic window films are incapable of changing from transmissive during winter months to reflective during summer months in order to effectively use electromagnetic radiation for dynamic temperature control of biological environments (e.g. human habitats, greenhouses and the like).
An alternative approach to electromagnetic radiation control involves using special glass panels having radiation transmission characteristics which effectively absorb (i.e. block) the infrared and ultra violet wavelengths, while transmitting the visible wavelengths thereby allowing window viewing and day light to enter the interior spaces of buildings using such window technology. While the light transmission characteristics of such glass provides a measure of electromagnetic radiation control during cooling seasons, where outdoor temperatures tend to be above 72 degrees Fahrenheit, its IR absorption characteristics prevents, during heating season, IR wavelengths of sunlight to warm the interior spaces of building structures in which such glass panels are installed. Consequently, during heating seasons, such glass fails to lessen the thermal loading on the heating systems of such buildings, as would be desired in an effort to conserve energy and heating resources during the winter months.
In recent times, there has been great interest in using variable light transmission glass or glazing, referred to as “smart windows”, to achieve electromagnetic radiation (i.e. energy) control in buildings and vehicles alike. The reason for using smart window structures, rather than conventional glass window panels, is quite clear. Smart window structures have light transmission characteristics that can be electrically controlled during the course of the day (or year) in order to meet lighting needs, minimize thermal load on heating and/or cooling systems, and provide privacy within the interior spaces of buildings and vehicles alike.
The use of chromogenic switchable glazing or smart windows for controlling the flow of light and heat into and out of a glazing according to occupant comfort, is discussed in great detail in the following papers: “Chromogenic Switchable Glazing: Towards the Development of the Smart Window” by Carl Lempert published in the June 1995 Proceedings of the Window Innovation Conference, Toronto, Canada; and “Optical Switching Technology for Glazings” by Carl Lempert published in Thin Solid Films, Volume 236, 1993, pages 6-13, both incorporated herein by reference.
In general, there are several different types chromogenic switchable glazing or smart windows, namely: non-electrically activated switchable glazings; and electrically-activated switchable glazings. The non-electrically activated types of chromogenic switchable glazing are based on: photochromics, thermochromics and thermotropics. The most common electrically-activated types of chromogenic switchable glazing are based on polymer dispersed liquid crystals (PDLC), dispersed particle systems (DPS), and electrochromics.
Prior art smart window structures based upon conventional twisted nematic (TN) or super twist nematic (STN) liquid crystal technology require the use of a pair of polarizers. This, however, results in high optical loss, as up to 60% of the incident light is absorbed by the polarizers, in the desired non-blocking mode of operation.
While a smart window structure based on polymer dispersed liquid crystal (PDLC) technology offers better performance than TN or STN based window structures, such smart window structures suffer from several significant shortcomings. Such electrochromic technologies are disclosed in greater detail in “Laminated electrochromic device for smart windows” by P. Schlotter, G. Baur, R. Schmidt, and U. Weinberg, P.351, Vol. 2255 (1994), and particle suspended technologies as disclosed in U.S. Pat. No. 4,663,083, entitled “Electro-optical dipole suspension with reflective-absorptive-transmissive characteristics” issued to Alvin M. Marks.
For example, when a voltage is applied to the electrochromic device in its “clear” state, it darkens as ions (such as lithium ions) and associated electrons transfer from the counter electrode to the electrochromic electrode layer. The tinting continues until the electrochromic system reaches its most opaque state. Reversing the voltage polarity causes the ions and associated electrons to return to the counter electrode, and the device becomes more transparent. However, the electrochromic device suffers from slow response time and shorter life-time. In particle suspended technology, the micro-sized dipole metal flakes are suspended in a carrier. When no electric field is applied, the particles are more or less randomly oriented. Therefore, the light is mostly reflected and/or absorbed, resulting in a low transmittance. When an electric field is applied across the device thickness, all the particles are aligned in the field direction. The device shows an optically transparent state. However, this technology has a problem associated with the settling of the metal particles due to gravity.
Using liquid crystal to make electrically controllable light devices has the promise to overcome these problems. These devices introduce a polymer matrix in liquid crystal materials that can be switched from translucent to transparent state by applying an electric field.
One known method of creating a switchable electro-optical device using stabilized liquid crystal structures is polymer dispersed liquid crystal (PDLC) technology as disclosed in “Polymer-Dispersed Liquid Crystals: Boojums at Work”, by J. William Doane, in MRS Bulletin/January, 1991. PDLC technology involves phase separation of nematic liquid crystal from a homogeneous liquid crystal mixture containing a suitable amount of polymer. The phase separation can be realized by polymerization of the polymer. The phase separated nematic liquid crystal forms micro-sized droplets dispersed in the polymer bed. All synthetic resins proposed before this invention are of the isotropic phase with an index np matching the ordinary index no of the nematic. In the off state, the liquid crystal molecules inside the droplets are randomly oriented. The mismatching of the refractive indices between the polymer bed and liquid crystal droplets causes the device to exhibit a translucent state, i.e., a light scattering state. When an electric field is applied, the liquid crystal orients in such a way that no=np resulting in a transparent state. The main disadvantage of the PDLC technology is the inherent haze caused by the optical index mismatching, particularly at large viewing angles.
The second problem associated with prior art PDLC technology is its high cost of manufacture. Virocon/3M (U.S.A.), and Raychem/Taliq (U.S.A.) are commercial manufacturers of privacy window glazing based on PDLC technology. Due to the extremely high price of manufacture, such manufacturers are facing significant obstacles in expanding the PDLC privacy window market.
U.S. Pat. No. 5,691,795 entitled “Polymer Stabilized Liquid Crystal Light Modulation Device and Material” by J. William Doane et al, incorporated herein by reference, discloses another liquid crystal technology based on liquid crystal polymer stabilized cholesteric texture (PSCT), which can be used to create electro-optical structures, such as electro-optical glazing structures. In PSCT technology, a small amount of UV cross-linkable polymer in its liquid crystal phase is mixed with cholesteric liquid crystal (CLC) whose pitch is tuned to the infrared region. The mixture is then cured by exposure to UV light while a voltage is applied to align the liquid crystal as well as the polymer molecules in the direction across the device thickness. After curing and when no electric field is applied, the liquid crystal material exists in a special cholesteric phase, i.e., a focal conic state. In this phase, the liquid crystal material exhibits a translucent state that is stabilized by the polymer network. When an electric field is applied, the CLC molecules are untwisted and aligned along the direction of the electric field, resulting in a transparent state. Since this technology requires much lower polymer concentration than that of PDLC technology and does not have liquid crystal droplets, it exhibits significantly lower haze, particularly when the refractive index of the polymer matches that of the cholesteric liquid crystal. However, this approach calls for polymerizable liquid crystalline material(s) to act as the polymer to stabilize the focal conic cholesteric phase.
Prior art PSCT technology has at least five significant problems which hitherto have neither been addressed or solved in a satisfactory manner.
First, PSCT technology imposes a high requirement on the selection of the polymer materials since liquid crystalline polymer that has a mesogenic group is needed as disclosed in U.S. Pat. No. 5,691,795, supra. Such a liquid crystal polymer material needs to be specially synthesized. Therefore, the cost of such a liquid crystalline polymer becomes extremely high, making the price of the PSCT device even higher than that of the PDLC.
Secondly, in typical PSCT systems, since monomers with mesogenic groups are used, the formation of the polymer network will partially alter the orientational order at each cross-linking site. Due to the presence of the mesogenic groups on the polymer network, the non-reactive liquid crystal molecules that are close to the network are now strongly anchored onto the network. To switch all liquid crystal molecules along the direction of the applied electric field, a strong field is needed. Such a field often brings about electric shorting problems. To avoid shorting, a switching electric field of modest strength is adopted by industry. However, the liquid crystal molecules close to the polymer network will not respond to a modest switching field, resulting in strong haze, particularly at large oblique angles.
Thirdly, scaling-up the panel size of PSCT-based devices has been very difficult in practice. To make the device in large sizes, the same lamination technology adopted in making the PDLC can not be used because the glass substrates themselves are used to support the PSCT structure as the PSCT material is basically in a liquid-gel-like state.
Fourthly, making a large-size uniform PSCT device is difficult because this lamination method cannot be used. Rather, a filling method is called for. However, when filling liquid crystal into a large size panel, the flow streaks of the liquid crystal and polymer mixture induce readily noticeable marks. Therefore, the resulting PSCT device appears very non-uniform.
Finally, the cost of glass substrates with conductive Tin Oxide layer coatings is very expensive when using PSCT-based technology. Also, the cost of plastic substrates with conductive Tin Oxide layer coatings is very expensive when using PDLC technology. Such factors contribute to the high price of electro-optical devices based on PDLC and PSCT technologies.
Accordingly, there is a great need in the art to improved means and ways of manufacturing large-size liquid crystal based electro-optical glazing structures at lower costs than that afforded by prior art manufacturing systems and methodologies.
Thus it is clear that there is a great need in the art for an improved form of variable light transmission glazing structures and methods and apparatus for making the same in a way which avoids the shortcomings and drawbacks of prior art technologies.